Remove polyfill.io - service no longer secure

See https://fossa.com/blog/polyfill-supply-chain-attack-details-fixes/

Author: cdrini

BookReaderDemo/demo-iiif.html

@@ -5,7 +5,6 @@
     <meta name="apple-mobile-web-app-capable" content="yes">
 
     <!-- WC dependencies -->
-    <script type="text/javascript" src="https://polyfill.io/v3/polyfill.min.js?features=es2015%2Ces5%2CglobalThis"></script>
     <script type="text/javascript" src="https://unpkg.com/lit@2.1.2/polyfill-support.js"></script>
     <script type="text/javascript" src="https://unpkg.com/@webcomponents/webcomponentsjs@2.2.10/webcomponents-bundle.js"></script>
 

BookReaderDemo/demo-internetarchive.html

@@ -8,7 +8,6 @@
     <meta name="apple-mobile-web-app-capable" content="yes">
 
     <!-- WC dependencies -->
-    <script type="text/javascript" src="https://polyfill.io/v3/polyfill.min.js?features=es2015%2Ces5%2CglobalThis"></script>
     <script type="text/javascript" src="https://unpkg.com/lit@2.1.2/polyfill-support.js"></script>
     <script type="text/javascript" src="https://unpkg.com/@webcomponents/webcomponentsjs@2.2.10/webcomponents-bundle.js"></script>